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[Bug ld/12291] "ld -r" doesn't work with mixed IR/non-IR objects


From: hjl.tools at gmail dot com
Subject: [Bug ld/12291] "ld -r" doesn't work with mixed IR/non-IR objects
Date: Tue, 13 Oct 2020 14:07:15 +0000

https://sourceware.org/bugzilla/show_bug.cgi?id=12291

--- Comment #6 from H.J. Lu <hjl.tools at gmail dot com> ---
(In reply to H.J. Lu from comment #2)
> The proposal is at
> 
> http://www.kernel.org/pub/linux/devel/gcc/lto/mixed-IR/mixed-IR.pdf

Here is the proposal:

Link with mixed IR/non-IR objects

* 2 kinds of object files
  o non-IR object file has
    * non-IR sections
  o IR object file has
    * IR sections
    * non-IR sections
    * The output of "ld -r" with mixed IR/non-IR objects should work with:
        o Compilers/linkers with IR support.
        o Compilers/linkers without IR support.
* Add the mixed object file which has
  o IR sections
  o non-IR sections:
    * Object codes from IR sections.
    * Object codes from non-IR object files.
  o Object-only section:
    * With section name ".gnu_object_only" and SHT_GNU_OBJECT_ONLY type
    on ELF:
    #define SHT_GNU_OBJECT_ONLY 0x6ffffff8      /* Object only */
    * Contain non-IR object file.
    * Input is discarded after link.
* Linker action:
  o Classify each input object file:
    * If there is a ".gnu_object_only" section, it is a mixed object file.
    * If there is a IR section, it is an IR object file.
    * Otherwise, it is a non-IR object file.
  o Relocatable non-IR link:
    * Prepare for an object-only output.
    * Prepare for a regular output.
    * For each mixed object file:
      * Add IR and non-IR sections to the regular output.
      * For object-only section:
        * Extract object only file.
        * Add it to the object-only output.
        * Discard object-only section.
    * For each IR object file:
      * Add IR and non-IR sections to the regular output.
    * For each non-IR object file:
      * Add non-IR sections to the regular output.
      * Add non-IR sections to the object-only output.
    * Final output:
      * If there are IR objects, non-IR objects and the object-only
      output isn't empty:
        * Put the object-only output into the object-only section.
        * Add the object-only section to the regular output.
        * Remove the object-only output.
  o Normal link and relocatable IR link:
    * Prepare for output.
    * IR link:
      * For each mixed object file:
        * Compile and add IR sections to the output.
        * Discard non-IR sections.
        * Object-only section:
          * Extract object only file.
          * Add it to the output.
          * Discard object-only section.
      * For each IR object file:
        * Compile and add IR sections to the output.
        * Discard non-IR sections.
      * For each non-IR object file:
        * Add non-IR sections to the output.
    * Non-IR link:
      * For each mixed object file:
        * Add non-IR sections to the output.
        * Discard IR sections and object-only section.
      * For each IR object file:
        * Add non-IR sections to the output.
        * Discard IR sections.
      * For each non-IR object file:
        * Add non-IR sections to the output.

> 
> It is implemented on hjl/lto-mixed branch at
> 
> http://git.kernel.org/?p=devel/binutils/hjl/x86.git;a=summary

It has been moved at:

https://gitlab.com/x86-binutils/binutils-gdb/-/tree/users/hjl/lto-mixed/master

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